This invention relates in general to vehicle braking and, more particularly, to an improved electro-fluidic braking apparatus for a vehicle and to a method of operation of the apparatus that will allow the vehicle to resume operation following a breakdown.
It has been known to use an electro-fluidic braking system in combination with a fluid pressure operated braking means. For example, see U.S. Pat. No. 3,709,564 by Jeffrey et al, incorporated herein by reference. In systems of this type, as used in the railroad industry, an electronic control unit includes circuitry for providing a friction brake control signal in response to which brake control pressure is applied. A pneumatic to electric transducer senses the effective brake control pressure to provide feedback information to the electronic control unit. The frictional brake control signal provides continuous blending of the combined friction and dynamic braking necessary to satisfy the overall effective brake demand signal.
The major problem with this type of system arises when an electrical failure occurs in either the electronic control unit or the electromagnetic transducer valve which controls development of pneumatic pressure in accordance with the output from the electronic control unit. When a failure occurs, an automatic full service brake application is established. To allow a return to service, it has been known to provide a manual changeover valve to convert the brake control system from an electropneumatic to an automatic-pneumatic mode of brake control. In this automatic-pneumatic mode, the variation of brake pipe pressure is effective to render a train operative on a "limp-in" basis to return to the station under control of the friction brake.
However, when this type of system is used on a train and the changeover must be made manually, a delay is imposed, during which time the train is inoperative and consequently may be in danger of being struck by another train. This is especially true where tight operating schedules must be adhered to.
The Jeffrey et al invention sought to overcome the above-described problem by providing a braking system in which a controller device was selectively operative to different control positions by the operator. In the Jeffrey et al system, pneumatic brake pipe pressure is varied in parallel with variation of an electrical signal, the signal employed being an analog P-wire signal. A service brake control valve device is subject to operation in response to a pressure differential across its control piston between brake pipe pressure and a control reservoir pressure. During automatic pneumatic mode of control, control reservoir pressure is maintained constant to serve as a reference against which the brake pipe pressure variation is effective to develop graduated service brake pressure by way of a double check valve device. During normal electropneumatic mode of control, electropneumatic application and release control valves are operative to provide brake pressure by way of the opposite input of the double check valve in response to digital control signals produced by an electronic control unit which is effective to sense the P-wire signal. The dynamic brake effectiveness and a pressure feedback signal corresponding to the effective friction brake produces continuous dynamic/friction brake blending.
In one embodiment of the Jeffrey et al invention, the electropneumatic application control valve controls fluid pressure communication between the control reservoir and a displacement reservoir to which the control chamber of a convention relay valve device is connected; and in a second embodiment, the electropneumatic application control valve controls fluid pressure communication between a source of supply and the control chamber of a conventional on/off type pilot control valve which, in turn, controls fluid pressure communication between control reservoir pressure and brake pipe pressure. In each embodiment, control reservoir pressure is dissipated at least to the value of brake pipe pressure in response to operation of the electropneumatic application valve to thereby nullify the effect of a brake pipe pressure reduction at the service brake control valve. Control reservoir pressure is prevented from reducing with the brake pipe, however, in consequence of the electropneumatic valve failing to respond to a brake application signal for any reason, thereby establishing control reservoir pressure effective to operate the service brake control valve in accordance with reduction of brake pipe pressure thereat to provide a back-up brake application under automatic-pneumatic brake control.
In the first-mentioned embodiment, a load control valve, subject to variations in the vehicle load condition, controls the level of fluid pressure to which a supply reservoir is charged. This supply pressure is connected to the relay valve, the maximum output of which is thereby limited in accordance with the vehicle load condition. In the latter embodiment, the load control valve supplies brake pressure directly, thus eliminating the need for a separate relay valve device as provided in the first embodiment.
In contrast to the invention of Jeffrey et al, in applicant's invention, the electropneumatic braking means and the pneumatic braking means are not both operative at the same time.